Special agents investigating the largest ever leak of classified US documents feared the soldier suspected of being responsible had colluded with foreign intelligence officials, it has emerged.

At a preliminary hearing in the case of Private Bradley Manning – the soldier in question – an army computer crimes specialist spoke of his initial concerns about an overseas agency, noting that Manning had "lived in the UK".

It was also disclosed for the first time that the former confidant who reported Manning to the authorities warned them of another hacker alleged to have been involved in the WikiLeaks case – a former employee of the US department of energy.

Special Agent Mark Mander told a military court in Fort Meade that Jason Katz boasted of helping the whistleblowing website decrypt a video clip depicting a US attack in Afghanistan that left scores of civilians dead.

Footage of the Garani incident of 2009 is alleged to have been supplied to WikiLeaks by Manning.

During the second day of a hearing to establish whether Manning, 24, is to face a full court martial, the court was told of the investigation that immediately followed his arrest.

Information led special agents to the Maryland house of Manning's aunt, whom he was in contact with while deployed in Iraq as an intelligence analyst.

In one conversation, Manning questioned her about the impact of what is alleged to be one of his most high-profile leaks – that of video footage of a US Apache helicopter killing 11 men in Iraq, including a Reuters journalist and his driver.

Mander told the court: "Prior to detainment in Iraq, he contacted her and asked about the Apache video, about how the release of the video was being perceived in the US.

"He contacted her to ask her to make a posting to his Facebook page referring to the Apache video."

Investigators made trips to his aunt's house, both before and after Manning's belongings had been shipped back from Baghdad.

In a basement where his personal items were kept, the investigating team found confidential documents.

"There was one memory card which contained information. Some of that information was classified," Mander said.

He also revealed investigators' initial fears that Manning could have been working with an overseas agency.

"There was a great deal of concern that there could be a foreign intelligence service involved," the witness said.

Manning's links to people and places outside the US were checked. "He had lived overseas in the UK," Mander told the court.

The witness also discussed an investigation into another individual whom they suspected of assisting WikiLeaks.

The name of Jason Katz was provided to authorities by Adrian Lamo, a convicted hacker who became Manning's confidant before turning him in.

Katz, who worked at a government laboratory in New York, had boasted to Lamo of his efforts to decrypt the Garani footage for WikiLeaks, Mander told the court.